the lunchbox?
The stale sandwiches?
Or do you mean that your non stale sandwiches that you are eating from the lunchbox have been tainted with the 'stale sandwich smell' from said lunchbox, or are you eating something else? I presume that you are not being rude.
You confuse me, yet it shouldn't bother me, hmmm perhaps I'm developing a crush on you

Mr Logic mode. 'They' have recently discovered that if you store plastic bottled water next to turps in bottles then the water will become tainted with turps, so the plastics are obviously permeable to some fumes/gases/vapours so it also figures that the thicker plastic of the sandwich box woould hold some fumes/gas/vapour.